This invention relates to a system for feedback control of the air/fuel ratio in an internal combustion engine, usually an automotive engine, which is to be normally operated with a lean mixture and which uses a conventional three-way catalyst for reducing exhaust emission. The control means includes means to intentionally decreasing the air/fuel ratio under predetrmined operating conditions of the engine.
Recent automotive engines have to satisfy severe requirements as to high power performance, low exhaust emission and good fuel economy all together. One approach to the solution of problems related to such conflicting requirements is operating the engine with a very lean air-fuel mixture under precise control of the fuel feed system.
For example, a lean combustion automotive engine system is described in "NAINEN KIKAN" (a Japanese journal), Vol. 23, No. 12 (1984), 33-40. This system includes an air/fuel ratio feedback control system, which uses an oxygen-sensitive solid electrolyte device as an exhaust sensor to detect the actual air/fuel ratio in the engine, and a three-way catalyst which catalyzes not only oxidation of CO and HC but also reduction of NOx. The output of the exhaust sensor used in this system becomes nearly proportional to the actual air/fuel retio over a wide range which extends from a slightly sub-stoichiometric ratio to an extremely super-stoichiometric ratio, so that feedback control of the air/fuel ratio can be performed with widely variable target value. As a typical example, the target value of air/fuel ratio in the feedback control system is 21.5 during steady-state operation of the engine and changes to 22.5 under gently accelerating conditions, to 15.5 under idling conditions and to a sub-stoichiometic value in the range of about 12-13 under full-load operating conditions.
The use of a very lean mxture is very effective in reducing the emission of NOx to a level that meets the current regulations. The three-way catalyst incorporated in the engine system exhibits its activities while the air/fuel ratio is decreased from a super-stoichiometric range. Conventional three-way catalysts use a combination of noble metals of the platinum group and usually include rhodium Rh as an essential element.
When a three-way catalyst is used in combination with an air/fuel ratio feedback control system for performing controlled lean-combustion, it is not seldom that the activities of the three-way catalyst deteriorate in a relatively short period of time.